1. Field of the Invention
The present invention relates to processes and materials useful for preventing the formation of Newton ring difraction patterns which occur when a transparent concave surface is placed against a plane transparent surface, and more particularly is concerned with the prevention of such Newton ring patterns when a photographic film transparency is mounted onto the outer surface of a smooth transparent cylinder as in color separation scanning of color transparencies in the printing industry.
2. State of the Prior Art
In modern printing of multi-color pictorial matter the colors of the original artwork or photographic transparency are separated by electronic scanning into four primary colors to make litho half tone films. Four such litho films, each associated with one color are used to determine the ink patterns on the printing paper such that when colored inks corresponding to the separated colors are applied according to the half tone litho images the original colors of the artwork or photograph are reconstituted on the printed sheet.
Typical color separation scanning equipment in use at the present time includes a cylinder of highly polished and optically transparent material such as glass or plastic which is caused to rotate in the scanner about its longitudinal axis.
The photographic transparency to be processed is mounted to the exterior cylindrical surface of the drum or cylinder and placed in the scanner. A light source moves axially within the cylinder so as to illuminate the rotating transparency, while a high resolution photo-sensing device scans the opposite side of the transparency to derive an electrical output indicative of the relative presence of each of the four basic colors which are normally selected to by cyan, magenta, yellow, and black. The electrical output of the photosensor is processed electronically and used to make a litho half tone film consisting of small dots in a dense pattern which determines the application of the colored inks during subsequent printing. Four such litho films are prepared. The dots of each half tone litho film are offset relative to the other four such that the four colors of ink are not superimposed during printing but instead are applied adjacently one to another. When the final printed image is viewed with the unaided eye, the adjacent colored dots blend into the desired hue determined by the relative presence of the four basic color inks.
The electronic scanners used in the industry at this time are capable of considerable magnification of the original photographic transparency, up to approximately two thousand times, adjustable by the operator. It is common therefore to use relatively small format transparencies, such as 35 mm film frames and enlarge these to half tone litho images of 8 by 11 inches. Minute flaws barely perceptible on the original transparency are thus greatly magnified and may make a particular set of litho films unusable. For example, small scratches, air bubbles, dust, etc. must all be carefully avoided and the process generally carried out under clean room conditions.
A particularly troublesome condition has been the formation of so-called Newton rings when the transparency is mounted to the transparent cylinder and which are magnified in the enlargement. The appearance of such Newton ring patterns normally make the resulting litho films unusable. To correct the condition, the transparency must be removed from the mounting cylinder and remounted. The industry has resorted to two presently known approaches to resolve this source of difficulty. One attempted solution has been the use of an oil mount which involves the formation of a thin film of oil, such as Paraffin oil, between the transparency and the outer surface of the transparent cylinder. This method requires considerable care in the application of the oil because very minute air bubbles tend to form if any unevenness is present in the transparency. Any such air bubbles are of course greatly magnified and make the resulting half tone litho images unusable. As presently practiced, oil mounting of transparencies involves the fastening of a sheet of clear acetate along one edge thereof parallel to the longitudinal axis of the mounting cylinder. The color transparency is positioned between the acetate sheet and the cylinder surface and a small quantity of oil is applied along one edge of the transparency. The transparency thus sandwiched between the clear acetate and the drum is passed through a device which includes a pressure roller which moves over the clear acetate so as to evenly spread the oil in a thin film between the transparency and the mounting cylinder. The opposite edge of the clear acetate is then secured to the mounting cylinder as by means of a strip of adhesive tape. The mounted transparency is then carefully examined with the aid of a magnifying glass to detect the presence of any air bubbles. If any are found, the mounting procedure must be repeated until a bubble free oil film is obtained. The cylinder with the transparency mounted thereon is then inserted between the rotating spindles of the color scanner and the color separation process is then allowed to proceed. Once completed, the transparency is separated from the cylinder which must then be cleaned with a suitable solvent to remove all traces of mounting oil. The oil mounting procedure is painstaking and cumbersome and requires cleaning of both the cylinder and transparency after the color separation process has been completed.
An alternate procedure involves the use of so-called Newton ring powder, such as the product sold under the mark Rutherford Contact-Aid by the Rutherford Machinery Company, division of Sun Chemical Company. This product is a fine white powder which is lightly applied to the surface of the transparency to be placed in contact with the outer surface of the mounting cylinder in the scanner. This powder is a dry material consisting of minute particles of optically opaque material which is interposed between the contact surfaces of the transparency and the cylinder and in effect acts as a spacer between these surfaces. This spacing of the smooth surfaces is sufficient to prevent the occurence of the Newton ring patterns and has been adequate in the past. With presently used electronic scanners capable of very high magnification, however, the use of such powdered materials becomes objectionable in that the individual grains are enlarged sufficiently by the scanner to become visible in the enlarged screen and to detrimentally affect the quality of the resulting image. In addition, it is difficult to apply the anti-Newton ring powder evenly over the transparency surface and in just the right quantity to avoid the ring patterns without obscuring the photographic image.
There is therefore a continuing need in the color separation industry for improvement in the methods available for preventing the appearance of such Newton ring patterns during the preparation of color separation masks in color scanners. Newton ring patterns are a known optical phenomenon which occurs when a concave transparent surface is pressed or mounted against a plane transparent surface. In the vicinity of the point of contact between the concave and plane surfaces a thin air space or film exists due to the slight separation between the two. The Newton rings are colored rings caused by the interference effects which occur between light waves reflected at the upper and lower surfaces of the air film separating the convex surface and the flat surface. It is believed that the presence of moisture encourages the occurence of Newton rings when color transparencies are mounted to the cylinder of the color scanner.